An engine loaded on a vehicle is generally subjected to feedback control so that the air-fuel ratio of exhaust becomes a target air-fuel ratio, based on the results of detection by an exhaust air-fuel ratio sensor (for example, an O2 sensor or a linear air-fuel ratio sensor (LAFS)) provided in an exhaust pipe.
However, if the amount of fuel remaining in a fuel tank is small, for example, fuel supply may be insufficient, and the exhaust air-fuel ratio may continue to be lean. If such a lean state of the exhaust air-fuel ratio persists, there is a possibility that the exhaust air-fuel ratio sensor will be erroneously judged to be abnormal, and the engine cannot be controlled appropriately.
A solution to the above-mentioned problems is a technology comprising switching judgment conditions for judging an abnormality in a system according to the amount of fuel remaining within a fuel tank, thereby preventing a temporary abnormal phenomenon, which occurs owing to a decrease in the remaining amount of fuel, from being erroneously judged to be an abnormality in the system (see Patent Document 1).